125W dissipation over 5 power resistors?

i expect your resistors to run hot

i expect the heat sink material is aluminum. while copper is marginally better, it isn't worth the cost,
and the increased difficulty if it needs to be machined

you're starting 10 deg C hotter than expected. (35 deg C instead of 25 deg C)

gravity is not sufficient. you want the resistors screwed down to make a good thermal contact.
you want to use a high quality pad or thermal grease.

(gravity instead of screws tells me this is a lab experiment -
you only need permission from your supervisor to run an experiment)

since no thermal resistances are provided for the resistors, nor is there a thermal resistance
for the preferred heat sink, i don't see any good way to get a quantitative answer

suggest you contact the manufacturer and find out the thermal resistances and etc.
then do the math

Appliance fans from room heaters can blow a quite large volume of air, and dissipate kilowatts to heat a whole room, so no problem there.
Coupling the heat into the airstrem with such a small plate might be the issue.
Normally in free air, 32 square inches might be good for about 32 watts with a surface temperature rise of about very roughly 60C.
With a serious air blower x8 or 256 watts might not be totally unrealistic, if the airflow is directed at high velocity on both sides of the plate. With fins there should be no problem at all.

I would not trust gravity, especially in a high air velocity !
Spend up big and buy some screws.

I've saved the guts out of some "ceramic" heaters for
power-resistor duty - they are "slabs o' fins" with some
resistor inside. Just disconnect the power feed and
the heater element with cooling fan is a good high (kW)
power load (if you happen to like the resistance ranges,
or have a plurality of them to series-parallel, which is
why the hoarding).

There's also types that still use coiled nichrome wire
that you could re-cut to specific R value (@ 25C or
@ final temp) or use a "sliding tap" with a jam screw
to make variable, fan cooled.

Warpspeed said:

Appliance fans from room heaters can blow a quite large volume of air, and dissipate kilowatts to heat a whole room, so no problem there.

Click to expand...

will hold the resistors on the plate....and some heatsink paste between the two.

Click to expand...

Considering that this plate will be populated with 5 hot devices, at the end you will have the equivalent of 25W (125/5) dissipating on a flat surface of only (20/5) 5cm x 10cm x 0.8cm. Even having being subjected to forced air, you must ensure a good thickness of thermally conductive material on the opposite side of the plate, just below the resistors, such as thick bars, if not adding heatsinks themselves; what dictates the ability to dissipate heat is precisely the surface available to spread it.

That's easy when you put all of them in distilled water tank or non-flame oild tank.
With water it will boil if temperature higher 100 degree. Risky is electrolysis.